Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial

ABSTRACT Objective: The primary objective was to compare round multi-strand wire and Ortho-Flex-Tech™ rectangular wire retainers in terms of gingival health. The secondary objectives were to assess plaque/calculus accumulation, and to determine the effectiveness of these retainers in maintaining tooth alignment and their failure rate. Material and Methods: This single-center study was a two-arm parallel randomized clinical trial and was conducted at the Orthodontic clinics in Dental Teaching Center/Jordan University of Science and Technology. Sixty patients, with bonded retention for the mandibular anterior segment after fixed orthodontic treatment, were randomly selected. The sample comprised Caucasian patients with mild to moderate pretreatment crowding in the mandibular anterior region, Class I relationship, treated without extraction of mandibular anterior tooth. In addition, only patients presenting normal overjet and overbite after treatment were included. Intervention: One group received round multi-strand wire retainer (30 patients, average age: 19.7 ± 3.8 years), while the other group received Ortho-Flex-Tech™ retainer (30 patients; average age: 19.3 ± 3.2 years). In both groups, the retainers were bonded to all mandibular anterior teeth from canine to canine. All patients were recalled one year after bracket debonding. Randomization sequence was created using Excel 2010, with a 1:1 allocation, using random block size 4. The allocation sequence was concealed in sequentially numbered, opaque and sealed envelopes. Only participants were blinded to the type of bonded retainer used. The primary outcome was to compare the gingival condition between the two groups. The secondary outcomes were to assess plaque/calculus indices, irregularity index of the mandibular anterior teeth and retainers’ failure rate. Comparisons were conducted using Mann-Whitney U test or chi-square test. Statistical significance was predetermined at the p≤ 0.05 level for all tests. Results: Complete data were collected for 46 patients (round multi-strand wire retainer group, n=24 patients; rectangular Ortho-Flex-Tech™ retainer group, n=22 patients). No significant differences were found in the gingival health parameters between the two groups (p>0.05). Ortho-Flex-Tech™ retainers maintained the alignment of mandibular anterior teeth more than multi-strand retainer (p<0.05). No significant difference was found in the failure rate between the two groups (p>0.05). Conclusions: Gingival health parameters and failure rate were not different in both groups. However, Ortho-Flex-Tech™ retainers were more efficient to retain the mandibular incisors than the multi-strand retainers; nevertheless, the difference was not clinically significant.

Al-Maaitah EF, Alomari S, Al-Nimri K -Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial

INTRODUCTION
Retention of teeth after active orthodontic treatment is usually recommended to overcome the potential of relapse, which can be variable and unpredictable. A Cochrane review found a lack of high-quality evidence to favor one method of retention over another, in terms of stability. 1 The duration of teeth retention has long been a dilemma in Orthodontics; however, long-term retention in the form of bonded retainer has been shown to be an effective way, in particular in the mandibular anterior segment, to minimize both relapse and maturational changes [2][3][4] with minimal patient compliance. 5,6 Two main designs of bonded retainers are currently in use: 1) rigid round wire bonded to the terminal teeth, which can be the first premolars or the canines; and 2) round multi-strand wire retainer bonded to all teeth in the anterior segment, usually from canine to canine. [7][8][9] Multi-strand stainless steel wire retainers are increasing in popularity, due to their flexibility, which allows for some physiologic tooth movement. 10 It has been reported that multi-strand wire retainers are more effective in maintaining individual tooth rotation, compared to the rigid wire retainers, whereas the latter were shown to be more hygienic. 11 Furthermore, rigid wire retainers showed less failure rate, compared to the multi-strand wire retainers. 12 Al-Maaitah EF, Alomari S, Al-Nimri K -Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial Many systematic reviews 1,13-15 found a lack of evidence to endorse the use of one type of orthodontic retainers based on their effect on: survival and failure rates, periodontal health, patient-reported outcomes and cost-effectiveness. Largely, these findings were attributed to a lack of high quality of relevant research and the high amount of methodological heterogeneity in study designs, types of wire used, methods of comparisons and outcomes reported. 14,15 A new design of bonded retainer, called Ortho-Flex-Tech™, was recently developed by Reliance Orthodontic Products, Inc. Because the Ortho-Flex-Tech™ retainer is gold-plated and allow for physiologic tooth movement, it is thus hypothesized that it has a more hygienic design and promote less hazard on the gingival health. Additionally, the cross-section of the Ortho-Flex-Tech™ retainer is rectangular in shape, when compared to round multi-strand wire retainer, which could provide more tooth contact surface area and maintain the alignment of the teeth better than the round single point contact wire retainer.

SPECIFIC OBJECTIVES OR HYPOTHESES
The primary objective of this randomized clinical trial was to compare round multi-strand wires and Ortho-Flex-Tech™ rectangular wire retainers, in terms of gingival health. The secondary objectives were to assess plaque and calculus accumulation, and to determine the effectiveness of these retainers in maintaining tooth alignment, as well as their failure rate. The null hypothesis was that there would be no difference between the two retainers, regarding gingival health, plaque accumulation, tooth alignment and failure rate.

STUDY DESIGN AND CHANGES AFTER TRIAL COMMENCEMENT
This single-center study was a two-arm parallel randomized clinical trial with a 1:1 allocation. The methods were not changed after initiation of the trial. Patients were asked to attend the clinic immediately within 24 hours, in case of any bonding failure or retainer fracture.  Simplified Oral Hygiene Index (SOHI) was assessed to compare the oral hygiene of the two groups. To measure the SOHI, four posterior and two anterior teeth surfaces were scored for debris and calculus accumulation, as described before by Greene and Vermillion. 16 The six surfaces examined were the buccal surface of the maxillary first permanent molars, the lingual surface of the mandibular first permanent molars, the labial surface of the maxillary right and the mandibular left central incisors. In the present study, the mandibular left central incisor was excluded, as the amount of debris and calculus may have been affected by the presence of the bonded retainer. Debris amount was scored on a scale of 0 to 3. The total debris score was divided by the number of surfaces scored for each patient. The same method was used to obtain the Calculus Index scores. The debris score and calculus score were combined to obtain the SOHI score.

Primary outcome
One year after bracket debonding, all subjects were recalled by the same clinician (S.A.), and the Gingival Index (GI) was recorded for the mandibular anterior teeth. To obtain the GI, the buccal, lingual, mesial and distal surfaces of the mandibular anterior teeth were scored from 0 to 3; these scores were averaged to obtain the GI for each tooth. 17 The total scores of the mandibular anterior teeth were divided by 4 to obtain the GI score for the mandibular anterior segment.
Al-Maaitah EF, Alomari S, Al-Nimri K -Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial

Secondary outcomes
The secondary outcomes assessed the plaque index (PI), calculus index (CI), simplified oral hygiene index (SOHI), and irregularity index (IRI) of the mandibular anterior teeth, and retainers' failure rate.
PI was determined by scoring the buccal, lingual, mesial and distal surfaces of the mandibular anterior teeth from 0 to 3, according to the amount of plaque on each surface. 17 The total score was divided by 4 to obtain the PI for the tooth. The PI score for the mandibular anterior segment was the average PI score of the mandibular anterior teeth.
The IRI was determined by using Little's irregularity index to measure the irregularity of the mandibular anterior teeth 18   The patients were asked to attend the clinic immediately within 24 hours if the retainer was debonded from any tooth or if the retainer was broken. Moreover, the patients were recalled on monthly basis, to check for any broken retainer not perceived by the patient.

Sample size calculation
Sample size calculation was performed using G*power v. 3.1.9.4 software, based on a previous study 11 . The effect size was calculated as the mean difference between the two groups, divided by the standard deviation of one group. In a previous study, 11 the effect size for the gingival health and plaque indices was 0.46, whereas for the irregularity index, it was 0.96. The calculation revealed that 44 patients were required (22 patients per group) to achieve a power (1-β error) of 80% at alpha level of 0.05. Sixteen patients were added to compensate for attrition rate of 25%.

Interim analyses and stopping guidelines
Not applicable.

Randomization (random number generation, allocation concealment, implementation)
Participants were randomly allocated to either multi-

Statistical analysis (primary and secondary outcomes, subgroup analyses)
Statistical analysis was performed using the Statistical Package for the Social Sciences software (SPSS v. 22.0, SPSS Inc., IL, USA).
Shapiro-Wilk w-test revealed that data were not normally distributed. Comparisons were conducted using Mann-Whitney U test or chi-square test; depending on the examined variable (numerical or categorical). Statistical significance was predetermined at the p ≤ 0.05 level for all tests.

Measurement error
To determine the measurement error, 10 subjects (5 subjects from each group) were randomly selected and re-examined by the same clinical examiner (S.A.) 7 days after the initial examination. The differences between first and second measurements were tested using the intraclass correlation coefficient for measurement error for the IRI.

BASELINE DATA
Baseline and pre-orthodontic treatment demographic and clinical characteristics for each group are presented in Table 1.
Baseline data for the gingival, calculus, plaque and simplified oral hygiene indices are expected to be deteriorated at the bracket debonding visit, due to fixed orthodontic treatment. Accordingly, the baseline data for these parameters were considered from the time of debonding; however, at this appointment, the teeth in all subjects were submitted to thorough scaling by the same clinician after bracket debonding, to improve these parameters and make the gingival health status for all included patients almost equal. Al-Maaitah EF, Alomari S, Al-Nimri K -Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial

SUBGROUP ANALYSES
Six (3 male, 3 females) of 30 patients from round Multi-strand wire retainer group, and 8 (6 male, 2 females) of 30 patients from Ortho-Flex-Tech™ wire retainer group were excluded from the study (Fig 3). The primary analysis was carried out per protocol, and complete data were obtained for 46 patients of the total 60 randomized sample: Multi-strand wire retainer group n=24, and Ortho-Flex-Tech™ wire group n=22.

PRIMARY OUTCOMES
The mean GI scores for each group are shown in Table 2. There was no significant difference in GI between the two groups. The mean GI for different teeth surfaces is shown in Table 3.
When the mean GI was divided into tooth surfaces, no significant differences were detected between the two groups. Table 2 shows the SOHI, PI, and CI mean scores for each group.

SECONDARY OUTCOMES
There was no significant difference in these parameters between the two study groups. The mean PI and CI for different teeth surfaces are shown in Table 3. When the mean PI and CI were Al-Maaitah EF, Alomari S, Al-Nimri K -Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial

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Dental Press J Orthod. 2023;28(2):e2321101 divided into tooth surfaces, no significant differences were detected between the two groups. IRI scores and the number of fractured retainers for each group are shown in Table 2.
The IRI was significantly higher in the Multi-strand wire group than the Ortho-Flex-Tech™ wire group one year after installing the retainer (p=0.048). However, the maximum mandibular incisor irregularity did not exceed 2 mm. The failure rate of multi-strand wire retainers was 8%, compared to zero failure rate of Ortho-Flex-Tech™ wire retainer group. However, this difference was not statistically significant (p > 0.05).

ERROR OF THE METHOD
Intra-examiner reliability was very good, as the intraclass correlation coefficient for measurement error for the IRI was 0.884.

HARMS
No negative outcomes were reported by any subject during the trial.

DISCUSSION
Long-term retention of mandibular incisor alignment using bonded retainers is considered safe, predictable, 19 acceptable to most patients and quite compatible with periodontal health. 5  To the best of our knowledge, this randomized clinical trial is the first to compare Ortho-Flex-Tech™ wire and multi-strand wire retainer in terms of gingival health, plaque accumulation, tooth alignment and failure rate.
Long-term retention with bonded retainers may present potential negative effects on periodontal health. 4 In this study, PI, GI, CI, and SOHI were measured one year after debonding the appliances, since it has been reported that a minimum follow-up period of 6 months is necessary to distinguish between gingival inflammation associated with fixed orthodontic treatment and that related to the orthodontic retainers. 20 No significant differences were detected between the two groups in this study in terms of PI, GI, CI, and SOHI. Even when the mean PI, GI and CI were divided into tooth surfaces, no significant differences were detected between the two groups. A recent systematic review with meta-analysis 15 concluded that orthodontic bonded retainers seem to be a retention strategy compatible with periodontal health, or at least not related to severe detrimental effects on the periodontium.
In the present report, the only statistically significant difference between the two groups was related to IRI. Ortho-Flex-Tech™ wire showed better alignment retention than multi-strand wire group. However, multi-strand wire group presented clinically less than 2 mm irregularity mean score. Previous studies have tried to define acceptable levels of relapse, and suggested that irregularity less than 3.5 mm in the anterior segment may be considered an acceptable level of relapse; 22,23 however, some patients would not tolerate this minor amount of irregularity. 4 Al-Maaitah EF, Alomari S, Al-Nimri K -Comparison between round multi-strand wire and rectangular wire bonded retainers: a randomized clinical trial

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Dental Press J Orthod. 2023;28(2):e2321101 Ortho-Flex-Tech™ wire retainer showed zero irregularity score, thus proving good efficiency. Ortho-Flex-Tech™ retainer has rectangular cross-sectional surface, compared to the round cross-sectional surface of the multi-strand wire retainer. This feature may give more control and prevent tooth from moving. However, a long-term study is still necessary to verify these results.
In the present study, the IRI of the multi-strand wire retainer group (0.69 mm) was almost similar to that reported by Årtun et al. 21 (0.66 mm) and lower than that reported by Al-Nimri et al. 11 (1.92 mm). No previous studies evaluated Ortho-Flex-Tech™ wire retainer in terms of maintaining the alignment.
Although the difference in the retainers' efficiency in this study was statistically significant, the clinical significance is questionable, as the mean difference in the IRI between the two groups was less than 1 mm. Long-term follow up may be necessary in the future to clarify this issue.
Failure of a bonded retainer is a relatively common complication, with prevalence ranging from 1 to 53%. 24,25 In this study, two retainers failed in the multi-strand wire group, compared to no failure in the Ortho-Flex-Tech™ group. However, this difference was not statistically significantly. Again, this may be attributed to the fact that both retainer types are bonded to all anterior teeth